Microwave ovens are common domestic appliances used for heating food. Generally they operate at a fixed frequency of 2.45 GHz, which is allocated for industrial use by national regulatory authorities and international agreement. It is desirable on the one hand to equip the oven with a window permitting observation of the food during heating and cooking, while it is necessary on the other hand to prevent harmful levels of microwave radiation from escaping from the oven, and potentially harming people in the vicinity of the oven. Today this is commonly accomplished by fitting the door of the oven with a double glazed window exhibiting a metal grid in the inter-pane region, or a by the use of a metal grid covered on both sides by plastic sheets. The metal grid is typically fabricated from a metal sheet, in which a multiplicity of small holes have been punched, or by using a woven or expanded metallic screen, characterized by a periodic array of openings separated by metal. Each hole or opening is much smaller than the wavelength of approximately 12.2 cm of the 2.45 GHz radiation, and thus the microwave power which escapes through the grid is greatly attenuated.
While these grids are effective in reducing the radiation to what has been determined to be safe levels, the visibility of the oven contents through the grid is generally poor. It is desirable to have an oven window with greater visibility, while providing adequate attenuation of the microwave radiation to meet safety standards.
Several inventions have been proposed to improve visibility using thin films which attenuate microwave radiation. U.S. Pat. No. 2,920,174 to Haagensen issued Jan. 5, 1960, hereinafter the '174 patent, the entire contents of which are incorporated herein by reference, teaches the use of thin metallic thin films to reflect microwave radiation while transmitting optical radiation. The '174 patent further teaches that the effective thickness of a metal film may be increased by metallizing opposed surfaces of a base member. Unfortunately, a practical microwave oven window utilizing inexpensive commercially available materials is not taught by the '174 patent.
U.S. Pat. No. 5,981,927 to Osepchuk et al. issued Nov. 9, 1999, the entire contents of which are incorporated herein by references, teaches the use of an absorbing film together with a metal screen. The requirement for a metal screen does not satisfactorily resolve the issue of visibility.
U.S. Pat. No. 6,822,208 issued Nov. 23, 2004 to Henze et al, the entire contents of which is incorporated herein by reference, teaches the use of a optically transparent microwave absorbing first film and an optically transparent microwave reflecting second film. Henze et al. intends for the first film to not only attenuate microwave transmission, but also to use the absorbed microwave energy to heat itself, and a transparent panel which supports it, and thus to prevent water condensation which could occlude visibility.
Microwave absorbing films have several disadvantages including: they absorb microwave energy intended for heating the contents of the oven; and in so doing, they, and the substrate supporting them, are heated, and can reach substantially elevated temperatures. Such elevated temperatures can constitute a safety hazard, since a user removing food or other contents from the oven might be injured touching the inside window. Furthermore, the periodic heating and cooling can compromise the integrity of the window by periodically stressing the interface between the film and the substrate and hence encouraging delamination of the film, and by producing thermal stresses in the substrate which exceed its yield strength, and hence causing the substrate to crack.
It should be noted that all materials, and in particular thin films, can simultaneously interact with microwave radiation in several ways, including by absorption, reflection, and transmission of the microwave radiation. Since all materials absorb microwave radiation to some degree, the term absorbing film as used herein is meant to describe a film where absorption is the primary interaction. Furthermore, it should be noted that the degrees of absorption, reflection, and transmission of a thin film, and specifically a film whose thickness is much less than the wavelength and skin depth at the radiation frequency of interest, are controlled primarily by a quantity known as the surface resistivity denoted as R, and R=ρ/d, where ρ is the resistivity of the thin film material (expressed in International Standard units of Ohm-meters), and d is the film thickness. R is usually expressed in terms of “Ohms per square” [Ω/□]. This is the resistance which would be measured between perfectly conductive electrodes fitted along the length of any two opposing sides of a square sample of the film of any size. The influence of R on the absorption, reflection and transmission for a simple idealized example of a plane wave normally incident on a planar film with infinite lateral extent, having a surface resistivity of R, is illustrated in FIG. 1, where the x-axis denotes surface resistivity in Ω/□ and the y-axis denotes the coefficient of absorption, reflection and transmission respectively. Curve 2 plots the absorption coefficient as a function of R, curve 4 plots the reflection coefficient as a function of R and curve 6 plots the transmission coefficient as a function of R. The power absorption, reflection, and transmission coefficients are given respectively by Equations 1-3:
                                          S            a                                S            i                          =                              (                          4              ⁢                              (                                  R                  η                                )                                      )                                              (                              1                +                                  2                  ⁢                                      (                                          R                      η                                        )                                                              )                        2                                              Eq        .                                  ⁢        1                                                      S            r                                S            i                          =                  1                                    (                              1                +                                  2                  ⁢                                      (                                          R                      η                                        )                                                              )                        2                                              Eq        .                                  ⁢        2                                                      S            t                                S            i                          =                              (                                          2                ⁢                                  (                                      R                    η                                    )                                                            1                +                                  2                  ⁢                                      (                                          R                      η                                        )                                                                        )                    2                                    Eq        .                                  ⁢        3            where η=377Ω is the impedance of free space, S is the power flux, and the subscripts i, a, r, and t refer to the incident, absorbed, reflected, and transmitted powers.
It should be noted that R is inversely proportional to the film thickness d, and thus a given electrically conductive material can act primarily as a transmitter, absorber, or reflector of microwave energy, depending upon its thickness. Thus a very thin film of electrically conductive material with a very large surface resistivity, e.g. R>377Ω/□, will primarily transmit incident microwave radiation, while a similarly constituted film of intermediate thickness such that 94Ω/□<R<377Ω/□ will primarily absorb incident microwave radiation, and a similarly constituted film of a greater thickness such that R<94Ω/□ will primarily reflect incident microwave radiation. While these numbers pertain to the specific idealized example examined, the principle here described is general. Henze et. al., for example, teach using a first film with a surface resistivity of 200Ω/□ denoted point 8 on FIG. 1. As may be seen in FIG. 1, this is the value of R yielding the largest absorption coefficient, 0.5.
The prior art teaches the use of various materials for thin films which are both optically transparent and electrically conductive, including metals, and in particular transparent conductive oxides such as indium tin oxide and various doped and undoped varieties of tin oxide and zinc oxide, as well as various techniques of depositing these thin films, including various wet chemical, physical vapor deposition, and chemical vapor deposition techniques. Some of these techniques are expensive to apply, while others yield poor adhesion or other properties. One technique in particular, however, atmospheric pressure chemical vapor deposition, applied in-line during the fabrication of float glass, provides good adhesion, good electrical and optical properties, and glass provided with this coating is commercially available at a relatively low price.
Thus, the prior art does not describe a low cost microwave oven window exhibiting good optical transmission. Furthermore, despite the long history of microwave ovens, a microwave oven with a suitable optically transparent window remains commercially unavailable,